The present invention relates to superconducting generators, and more particularly to a combined damper and shield for generator rotors having superconducting field windings.
Large alternating current generators with superconducting field windings offer great potential benefits. Such machines operate with no losses in the field winding itself and thus can provide higher magnetic fields than conventional generators and with greater efficiency. This allows the generation of more power per unit volume of the generator and thus makes it possible to achieve greatly increased ratings or to reduce substantially the size and cost of a machine of given rating. The reduction in electrical losses during operation of the machine is also an important advantage and represents a very substantial saving in operating cost.
Such machines have rotating field members consisting of a cylindrical rotor with the field winding disposed in slots in its surface. The field winding conductors are made of superconducting material and means are provided for circulating a cryogenic coolant fluid during operation of the machine to maintain the winding in the superconducting state, that is, at a temperature below the critical temperature of the material. An external refrigeration system is provided for this purpose to supply liquid helium to the winding during operation, the helium exhausting in the gaseous state for return to the refrigerator.
In such a rotor, it is necessary to protect the superconducting field winding from alternating current magnetic fields in the air gap of the machine which may be caused by harmonics in the armature magnetomotive force and by unbalanced loads on the generator. Such fields, as well as the large transient fields that may occur during faults, could cause large eddy current losses in the field winding and produce sufficient heat to result in loss of superconductivity. Shielding is, therefore, required to protect the winding and can be provided by a cylindrical shield of high electrical conductivity surrounding the rotor and rotating with it. During disturbances on the system to which the generator is connected, the rotor will tend to oscillate at a low frequency about its steady-state torgue angle and damping must be provided to attenuate these oscillations in order to return to steady-state operation. This damping function can be performed by the same high-conductivity cylindrical member, which thus performs the two functions of damping and shielding.
The mechanical design of such a combined damper-shield presents difficult problems. The shield must have high electrical conductivity and it must also have sufficient mechanical strength to withstand the large forces to which it is subjected under fault conditions. In addition, the damper-shield and the rotor must be capable of withstanding the extremely high torques that may occur during a fault and the drive shaft which drives the rotor should be protected against these dangerously high torques. The damper-shield must also be mounted on the rotor in a manner which permits relative movement to allow for the differential thermal contraction of the rotor body when it is cooled to cryogenic temperature for operation, and to permit some relative axial movement between the damper-shield cylinder and the rotor body which results from different bending deflections during operation.